Inside the pages of your book may be a hidden image. And no, we're not talking about folding the pages of MAD Magazine. The University of Iowa has uncovered secret paintings in the edges of early 19th century books.
The technique is called fore-edge painting and actually dates back as early as the 1650s. The painting is visible on the edges of the book pages only when they are fanned back. The actual book edge is then gilded, making the painting invisible to anyone not in the know.
Bend the pages over just slightly. See the painting?
Here is a gif from the University of Iowa's research team of a fore-edge painting from 1837.
Often the fore-edge paintings served as illustrations to the text.
See how it completely disappears into the gilded edge when it's closed? Amazing.
These fore-edge books are valuable collector's items for rare book enthusiasts. You can follow the University of Iowa's team on their Tumblr where they document work on the library's special collection.